1. Field of the Invention
The disclosed technology generally relates to image-capturing methods, and more particularly to exposure metering methods for providing foreground and background of image frames that have stable and balanced brightness values. Aspects are also directed to apparatuses configured for the methods.
2. Description of the Related Art
Some digital imaging apparatuses, e.g., digital camcorders are equipped with auto exposure metering processes to enhance the visual effect of the images taken. The exposure metering processes are designed to continuously determine and adjust exposure conditions to be used by an image sensor of the digital imaging apparatus. The exposure conditions are determined by parameters including, among other parameters, time, gain, and aperture (f-number). Many digital images that are generated using conventional exposure metering processes can have unbalanced relative brightness values between an image foreground (e.g., an image subject) and an image background (e.g., the image minus the image subject). That is, one of the foreground or the background may appear unrealistically bright or “washed out” compared to actual human visual experience, while the other of the foreground or the background may appear unrealistically dark compared to actual human visual experience. For example, when a digital image of a person's face is taken against a background of a rising or a setting sun at a beach, the background may be washed out, while the foreground (i.e., the person's facial features) appears overly dark. On the other hand, in an opposite-type situation, such as when a digital image of a person's face is taken using a flash against a background of an evening party, the foreground (i.e., the facial features) may be washed out, while the background appears overly dark.
Such an imbalance between the foreground and the background of the image frames can sometimes be attributed to an exposure metering process that controls the exposure conditions of the digital imaging apparatus. Generally, exposure conditions are determined parameters including, among other parameters, the aperture, the shutter speed, and ISO speed. The aperture controls the area over which light can enter the image sensor of the digital imaging apparatus. The shutter speed controls the duration of exposure of the image sensor. The ISO speed controls the sensitivity of the image sensor to a given amount of light. While different combinations of the aperture, the shutter speed and the ISO speed can be used to achieve the similar exposure, different trade-offs can exist with the different combinations. This is because the aperture can affect the depth of field, the shutter speed can affect the motion blur and the ISO speed can affect the image noise. Some digital imaging apparatuses are equipped with standardized metering processes that autoselect some or all of the aperture, the shutter speed, and ISO speed.
Some digital imaging apparatuses are equipped with one or more exposure metering options, many of which work by assigning a relative weighting to different light regions. For example, in partial or spot metering, a relatively higher weight is assigned to a portion of the image, e.g., a face, to image the portion (e.g., the face) against a substantially brighter or darker background without rendering the portion overly dark or overly bright. However, while such metering processes may allow for a stable foreground (e.g., the face) with a desired brightness level by metering off at least a portion of the foreground, the metering processes still may not prevent the background from becoming overly bright or overly dark by the very exposure conditions that allow for the stable foreground with the desired brightness level.